Radio access technologies for wireless communication network are continuously being evolved to meet the future demands for higher data rates, improved coverage and capacity. Examples of recent evolutions of the Wideband Code Division Multiple Access (WCDMA) technology are High-Speed Packet Access (HSPA). Currently further evolutions of the 3G systems, 3G Long Term Evolution (LTE), including new access technologies and new architectures are being developed within the 3rd Generation Partnership Project (3GPP) standardization body.
One of the main targets for LTE is that the access technology should be flexible to use in existing frequency allocations and new frequency allocations in order to allow for easy introduction in spectrum with existing deployments. Also, it should be possible to use different duplex solutions. Both Frequency Division Duplex (FDD) and Time Division Duplex (TDD), where up- and downlink are separated in frequency and in time respectively will be supported to provide usage in unpaired spectrum. To allow for flexible spectrum solutions the access technology chosen is based on Orthogonal Frequency Division Multiplexing (OFDM) for the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) for the uplink.
The LTE concept supports fast scheduling in frequency and time both for the uplink and the downlink. This means that the resource assignment in time and frequency can be adjusted to the user equipments' momentary traffic demand and channel variations.
In the LTE uplink it is possible to scheduling several user equipments in one Transmission Time Interval (TTI) by assigning different frequency segments to different user equipments. To maintain the single carrier structure, each user equipment can only receive contiguous assignment in frequency. The resource assignment is performed by the scheduler situated in the base station, eNodeB. The scheduler informs the user equipments about the scheduling decision by transmitting grants containing resource assignments on the Physical Downlink Control Channel (PDCCH). A limited number of grants can be transmitted on this channel.
The LTE uplink is almost orthogonal. Due to transmitter and receiver imperfections there may be intra-cell interference in the form of mirror frequency interference and adjacent user equipment interference.
The scheduler manages the radio resource in frequency and time. In each scheduling time interval, the scheduler allocates a number of radio frequency resources to user equipments in the cell. When there are more than one user in the cell, the scheduler may need to solve the following problems and make a scheduling decision: Who or which users are selected to assign the radio frequency resources? How to divide a number of available radio frequency resources among a number selected users? How to place the selected users in frequency bandwidth or which part of frequency band is suitable for which users?
There are commonly used approaches or methods to solve these problems, which are described briefly below. The problems when using these approaches in the uplink scheduler are also addressed.
For the first problem, there are many approaches to select users when to allocate resources. Basically, there are two principles: the random selection or the priority based selection. With the random selection the scheduler selects user randomly when the resources are assigned. The random selection is not much useful when the quality of service (QoS) is a requirement in resource management. The priority based resource allocation is more commonly used approach in wireless communication network. In this approach, the users are queued according to a pre-defined priority weight function. The weight function can take many requirements, such as the QoS, the queuing time and the user throughput, etc., into the consideration. The scheduler selects user equipment one by one from the priority queue.
When a user is selected either randomly or from the priority queue, the scheduler may need to decide how many resources units should be assigned to the selected user. There are mainly two alternatives: either assigns one resource unit per selected user or assigns a number of resource units per selected user. The approach with randomly selected user usually applies the first alternative. This scheduling approach is known to be the Round Robin algorithm: where the scheduler assigns one resource unit to one selected user, and next resource unit to the next selected user until all resources are assigned.
The approach with priority based selection usually applies the second alternative. When the highest prioritized user is selected, the scheduler allocates as many frequency resources as user equipment needs first. If there are more resources left, the scheduler allocates the remaining resources to the second user equipment in the priority queue, and goes on until all resources are assigned.
The random selection approach is not appropriate in the wireless communication network since the QoS is one of the most important requirements when resources are allocated by the scheduler. The priority based approach has been a preferred solution and commonly used by the scheduler in the wireless communication system. It may function in the downlink when the base station has enough power to keep the channel quality. However when applying such an approach in the uplink scheduler, it is no longer appropriate when the power limitation of the user equipment or the impact of transmitter and receiver imperfections are taken into account. The following problems have been observed when this approach is applied in the uplink scheduler:
Firstly, the prioritized user equipment that is scheduled first, with a generous amount of frequency resource units or blocks, may not have enough power to transmit on the assigned resource units. Since the user equipment has to distribute the total transmission power to all assigned frequency resource units. The more assigned resource units the less power can be used on each resource unit, and hence the lower received power density, i.e. received power per frequency unit. Low received power density gives low channel quality; hence poorer channel utilization. This can result in very low throughput and bad performance both on the user equipment and the wireless communication system.
Secondly, when several user equipments are scheduled, the differences in received power density or Signal to Interference and Noise Ratio (SINR) density can be very large between scheduled user equipments. In some cases, the inter-frequency interference can be a problem.
For the second problem, the commonly used method to divide a number of available resources to a number of user equipments is to formulate such a problem into an optimization problem, and then solve the optimization problem by searching through all combinations or alternative choices. The searching of an optimal solution can be very time consuming in a large system with many user equipments and many resource units, as the number of possible combinations increase. Similarly, more available resources leads to more combinations. The number of combinations increases exponentially.
In the downlink, when power limitation may not be a big problem and when there may be no restriction on which resource unit can be assigned to the user equipment, the frequency dependent resource allocation can be used to place the selected user in the frequency units with the best radio channel condition. However, the solution is difficult to apply to the uplink scheduler. In the uplink, the user equipments are spread out geographically in a cell. The radio signals sent from the user equipment and received at the base station, i.e. uplink signals can be very different in power strength in different frequency bends, depending on radio propagation conditions such as e.g. the distance, the fast and slow fading between the user equipment and the base station. The received power density, which depends on the transmission power and radio propagation conditions, can be very different for different placement in frequency bands. The user equipment may be power limited in one frequency band placement and not power limited in other frequency bands. Different frequency bands also flaw different channel quality and hence different modulation and coding scheme (MCS) should be used. The user equipment may transmit all data in the data buffer in some frequency band placement and may not be able to transmit all data in other frequency band placement. Moreover, the inter-frequency Interference can also be a problem when two user equipments are placed in the neighbour frequency band with each other, one has a very low received power density, e.g. the user is power limited or data limited, and the other has a very high received power density. There is no systematic method to cover all these aspects in the uplink.
Even if the scheduler is able to find solutions to each problem listed above, the scheduler still needs to put all the solutions together and make a scheduling decision. Since there are interactions and conflictions of the problems, the solution that may solve one of the described problems may not solve the other problems the scheduler has to solve. The scheduler has to go back to find another solution for the solved problem and make a test on the other problems. The process can go on and get more and more complicated, which is not applicable in practice.